1. Field of the Invention
This invention relates to a radiation image read-out apparatus, and more particularly to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by detecting stimulated emission emitted from the stimulable phosphor sheet upon exposure to stimulating light by the use of a line sensor.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation such as X-rays, α-rays, β-rays, γ-rays, electron beams, ultraviolet rays and the like, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating light such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as “a stimulable phosphor”. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as “stimulated emission”. There has been known a radiation image recording and reproducing system in which a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) is exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet as a latent image, stimulating light such as a laser beam is projected onto the stimulable phosphor sheet, and the stimulated emission emitted from the stimulable phosphor sheet is photoelectrically detected, thereby obtaining an image signal (a radiation image signal) representing a radiation image of the object. See, for instance, Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395 and 56(1981)-11397.
As a system for detecting stimulated emission emitted from the stimulable phosphor sheet, there has been known a system in which stimulating light which is emitted from a stimulating light source as a divergent light bundle is converged on a line-like area on the surface of the stimulable phosphor sheet by a cylindrical lens and stimulated emission emitted from the line-like area of the stimulable phosphor sheet is detected by a line sensor comprising a plurality of linearly arranged photoelectric convertor elements.
However, in the case where the stimulating light is converged on a line-like area of the stimulable phosphor sheet (e.g., of a width not wider than 50 μm) by the use of a cylindrical lens, there is a fear that the line-like area becomes wider than a desired width or nonuniform in width due to false spherical aberration or false curvature of field of the cylindrical lens. An attempt to correct false spherical aberration and/or false curvature of field of the cylindrical lens to precisely converge the stimulating light on the line-like area by adding, for instance, a spherical lens increases the optical path length and increases the overall size of the apparatus.
The “false spherical aberration” means a stimulating light collecting error in a plane normal to the longitudinal direction of the cylindrical lens (the direction in which the cylindrical lens extends), that is, a plane in which the lens has refractive power, and the “false curvature of field” means an error in positioning of focal points on which the stimulating light is to be collected and which is to be arranged in the plane of refractive power along the longitudinal direction of the cylindrical lens, where the focal points are arranged on an arcuate line curving along the longitudinal direction of the cylindrical lens.